Could be a bad cable or connector. I just had a intermediate floor call button not working on a tac 20. I pulled the fixture off and pulled the 10 pin plug off the l24 card. I touched the pins together and the thing worked. Plugged back in and it didn't. Looked again and one pin was bent over and not going into plug. No idea how that happened without someone pulling the fixture and screwing it up but that was on the discreet wire plug. I mean also the spi loop plugs or wire could be bad